One method for modulating an RF signal with a digital data stream utilizes a digital modulator that accepts the digital baseband inputs, typically in a real and imaginary format, and outputs a digital signal at an IF sample rate. The digital IF signal is then converted to an analog signal by a digital-to-analog converter (DAC). The analog IF signal is mixed with a local oscillator to generate a modulated RF carrier.
These modulators require some form of equalizer to compensate for the variation in gain with frequency. In general, the required equalization will not be symmetric about the IF carrier frequency. For example, one source of nonsymmetrical gain roll-off associated with a digital modulator is the sin(x)/x response of the DAC used to produce the IF waveform from the digital signal. In addition, the response of amplifiers, filters and mixers in the IF or the following RF chain may also have gain functions that require equalization. The equalization compensates for these post IF gain functions by distorting the IF signal such that the distortions in the IF signal cancel the distortions introduced by the post IF components,
In prior art modulators, the equalization filter is placed either after the DAC, before the DAC, or before the digital modulator. If the filter is placed after the DAC, a complicated analog filter design that is not reprogrammable or adaptable to changes in the transmitter response is required.
In principle, the baseband data can be pre-emphasized to provide the required correction to the non-ideal frequency response of the channel. In such systems, the baseband data is distorted such that the combination of the introduced distortion and the distortions of the channel cancel each other. In this case, a simpler analog filter can be used after the DAC. However, since the required response is, in general, not symmetric about the IF carrier, a filter with complex coefficients or real-imaginary and imaginary-real cross terms is required. At frequencies near the Nyquist limit for the baseband signals, the correction filters that provide the desired response are not always achievable.
The third approach utilizes a digital filter between the digital modulator and the DAC. While this approach simplifies the filter design, it requires a digital filter that runs at the DAC sampling rate. At high IF frequencies, such filters are too expensive for many applications.